The present invention relates to a magnetically anisotropic rare earth-based permanent magnet having a nanocomposite structure either in the form of a block or in the form of a powder as well as to a method for the preparation thereof.
As is known, several classes of rare earth-based permanent magnets are now under industrial mass production and widely employed as a high-performance permanent magnet including those based on a samarium-cobalt alloy and those based on a neodymium-iron-boron alloy, referred to as Nd/Fe/B magnets hereinafter, as the major current. Of these two classes of the rare earth-based permanent magnets, the demand for the latter class magnets is rapidly growing as compared with the earlier developed former class of the magnets by virtue of their high magnetic properties and economical advantages due to the low material costs as compared with the magnets of the former class.
Several different processes have been developed for the preparation of the Nd/Fe/B magnets, of which the most widely practiced industrial method is the so-called sintering method. The Nd/Fe/B magnets prepared by the sintering method have a composite phase structure consisting of a hard magnetic phase of Nd.sub.2 Fe.sub.14 B in combination with subsidiary phases including, one, a phase rich in the content of neodymium and, the other, a phase rich in the content of boron as Nd.sub.1.1 Fe.sub.4 B.sub.4.
This Nd/Fe/B magnet by the sintering method is prepared powder-metallurgically in the following manner as reported by M. Sagawa, et al. in Japanese Journal of Applied Physics, volume 26 (1987), page 785. In the first place, namely, the respective starting materials of neodymium, iron and boron each in an elementary form are taken each in a specified proportion and melted together to form an alloy in such a weight proportion that the contents of neodymium and boron in the resultant Nd/Fe/B alloy are each somewhat higher than the stoichiometric content in Nd.sub.2 Fe.sub.14 B and the thus obtained alloy ingot is pulverized in an atmosphere of an inert gas into a fine powder of which the particle diameter is a few micrometers. The alloy powder is then compression molded into a powder compact or green body in a magnetic field so as to have the easy magnetization axes of the particles aligned in the direction of the magnetic field applied thereto followed by sintering by heating the green body in an inert atmosphere at a temperature of about 1000.degree. C. and an aging treatment at a lower temperature. The Nd/Fe/B magnet thus prepared is magnetically anisotropic by virtue of the cleaning effect on the interface of the principal phase of Nd.sub.2 Fe.sub.14 B by the Nd-rich phase surrounding the principal phase.
On the other hand, a process of so-called melt-spun method as reported by R. W. Lee in Physics Letter, volume 46 (1985), page 790 and elsewhere is known, in which a melt of a Nd/Fe/B alloy having a chemical composition identical to the target composition of the Nd/Fe/B magnet is ejected at the surface of a cold roller rotating at a high revolution to be rapidly solidified into a quenched thin ribbon of the alloy having an amorphous structure which is processed into a Nd/Fe/B magnet. Although the principal phase of the magnet of this type is also Nd.sub.2 Fe.sub.14 B, a relatively high coercive force can be obtained therein as a consequence of the small crystallite diameter in the nanometer range from 20 to 100 nm to be about equivalent to the diameter of a single magnetic domain as compared with the magnets prepared by the sintering method.
The Nd/Fe/B magnets prepared by the melt-spun method can be classified into three types. The magnet of the first type is the so-called bond magnet which is prepared by molding a composite blend consisting of fine particles obtained by pulverizing the quenched thin ribbons of the magnetic alloy and a resinous binder. Though advantageous in respect of the simplicity of the preparation process, bond magnets in general have a problem that the magnetic properties thereof cannot be vert high as compared with the magnets of the other types because the magnet is magnetically isotropic without alignment of the easy magnetization axes of the magnet particles as an inherence of the molding process in addition to the relatively low packing density of the magnet particles in the composite blend with the resinous binder. The second type of the Nd/Fe/B magnets prepared by the melt-spun method includes those isotropic bulk magnets obtained by hot-press molding of the fine particles obtained from the quenched thin magnetic ribbons. The Nd/Fe/B magnet of the third type, as disclosed in Japanese Patent Kokai 60-100402 and elsewhere, is obtained by subjecting the second type magnet to hot working so as to accomplish alignment of the easy magnetization axes of the magnetic particles along the direction of compression.
On the other hand, further efforts have been effected for developing high-performance rare earth-based permanent magnets of the next generation resulting in the debut of the now highlighted so-called nanocomposite magnets reported in IEEE Transaction Magnetics, volume 27 (1991), page 3588 by E. F. Kneller, et al. and elsewhere.
Each of the above described Nd/Fe/B magnets prepared by the sintering method and melt-spun method contains a hard magnetic phase of Nd.sub.2 Fe.sub.14 B as the principal phase but is free from any soft magnetic phases with an increased content of iron such as the phases of bcc-iron (body-centered-cubic iron), Fe.sub.3 B, Fe.sub.2 B and the like.
In contrast thereto, the newly developed nanocomposite magnets have a composite structure consisting of a hard magnetic phase and a soft magnetic phase finely and uniformly dispersed each in the other in a fineness order of several tens of nanometers. In such a nanocomposite structure of the magnet, it is understood that coupling is established between magnetization of the hard magnetic phase and magnetization of the soft magnetic phase by the exchange interaction so that reversal in the magnetization of the soft magnetic phase is inhibited resulting in a behavior of the magnet structure as a whole something like a single hard magnetic phase. The applicability of the above described principle of the nanocomposite magnet is not limited to newly developed magnetic materials but there is a possibility that even a conventional magnetic material can be processed into a nanocomposite magnet having a still higher saturation magnetization without causing a decrease in the coercive force. For example, it is reported by R. Skomeski, et al. in Physical Review, volume B 48 (1993), page 15812 that their theoretical calculation indicates that a samariumbased permanent magnet having a composite composition of the formula Sm.sub.2 Fe.sub.17 N.sub.3 /(Fe,Co) and imparted with magnetic anisotropy may have a maximum energy product (BH).sub.max of as high as 137 MGOe when the magnet has a nanocomposite structure.
Besides the above described theoretical studies, experimental results are reported for rare earth-based permanent magnets of the nanocomposite structure with several different combinations of the hard and soft magnetic phases including Nd.sub.2 Fe.sub.14 B/Fe.sub.3 B disclosed by R. Coehoorn, et al. in Journal de Physique, volume 49 (1988), page C8-669, Nd.sub.2 Fe.sub.14 B/Fe disclosed in Japanese Patent Kokai 7-173501 and 7-176417, Journal of Applied Physics, volume 76 (1994), page 7065 by L. Withanawasam, et al. and elsewhere, and Sm.sub.2 Fe.sub.17 N.sub.3 /Fe disclosed In Journal of Magnetism and Magnetic Materials, volume 124 (1993), page L1 by J. Ding, et al.
In each of these recently developed methods for accomplishing a finely dispersed structure of a rare earth-based permanent magnet, microcrystallites are formed by undertaking a heat treatment of the quenched thin ribbons obtained by the melt-spun method or particles of an amorphous alloy prepared by the mechanical alloying method. Accordingly, like the first type method mentioned above, the particles of the alloy cannot be magnetically aligned relative to the crystallographic orientation so that the permanent magnets obtained by these methods as such are necessarily limited to magnetically isotropic ones. Thus, no prior art is known on the process for the preparation of a magnetically anisotropic rare earth-based nanocomposite permanent magnet notwithstanding the theoretical studies suggesting possibility of obtaining such a high-performance permanent magnet.